In mining operations, the efficient fragmentation and breaking of rock by means of explosive charges demands considerable skill and expertise. In most mining operations explosive charges are planted in appropriate quantities at predetermined positions within the rock. The explosive charges are then actuated via detonators having predetermined time delays, thereby providing a desired pattern of blasting and rock fragmentation. Traditionally, signals are transmitted to the detonators from an associated blasting machine via non-electric systems employing low energy detonating cord (LEDC) or shock tube. Alternatively, electrical wires may be used to transmit more sophisticated signals to and from electronic detonators. For example, such signaling may include ARM, DISARM, and delay time instructions for remote programming of the detonator firing sequence. Moreover, as a security feature, detonators may store firing codes and respond to ARM and FIRE signals only upon receipt of matching firing codes from the blasting machine. Electronic detonators can be programmed with time delays with an accuracy of 1 ms or less.
The establishment of a wired blasting arrangement involves the correct positioning of explosive charges within boreholes in the rock, and the proper connection of wires between an associated blasting machine and the detonators. The process is often labour intensive and highly dependent upon the accuracy and conscientiousness of the blast operator. Importantly, the blast operator must ensure that the detonators are in proper signal transmission relationship with a blasting machine, in such a manner that the blasting machine at least can transmit command signals to control each detonator, and in turn actuate each explosive charge. Inadequate connections between components of the blasting arrangement can lead to loss of communication between blasting machines and detonators, and therefore increased safety concerns. Significant care is required to ensure that the wires run between the detonators and an associated blasting machine without disruption, snagging, damage or other interference that could prevent proper control and operation of the detonator via the attached blasting machine.
Wireless detonator systems offer the potential for circumventing these problems, thereby improving safety at the blast site. By avoiding the use of physical connections (e.g. electrical wires, shock tubes, LEDC, or optical cables) between detonators, and other components at the blast site (e.g. blasting machines) the possibility of improper set-up of the blasting arrangement is reduced. Another advantage of wireless detonators relates to facilitation of automated establishment of the explosive charges and associated detonators at the blast site. This may include, for example, automated detonator loading in boreholes, and automated association of a corresponding detonator with each explosive charge, for example involving robotic systems. This would provide dramatic improvements in blast site safety since blast operators would be able to set up the blasting array from entirely remote locations. However, such systems present formidable technological challenges, many of which remain unresolved. One obstacle to automation is the difficulty of robotic manipulation and handling of detonators at the blast site, particularly where the detonators require tieing-in or other forms of hook up to electrical wires, shock tubes or the like. Wireless detonators and corresponding wireless detonator systems may help to circumvent such difficulties, and are clearly more amenable to application with automated mining operations.
However, the development of wireless blasting systems presents new challenges with regard to safety issues. In one example, each wireless detonator assembly must include some form of communication means to allow the receipt, and processing by the wireless detonator assembly of command signals (e.g. ARM, DISARM, FIRE signals etc.) received wirelessly from an associated blasting machine, and optionally the transmission of signals (e.g. including status information, firing codes, delay times etc.) back to an associated blasting machine. For this purpose, each wireless detonator assembly must include some form of independent power supply (an “operating power supply”) sufficient to power the signal receiving, processing, and transmission components of the assembly. However, the presence of the operating power supply itself presents an inherent risk of inadvertent detonator actuation resulting from accidental or inappropriate application of the operating electrical power to the firing circuitry. This problem is recognized in the art, and several systems have previously been developed to reduce the risk of inadvertent detonator actuation.
For example, U.S. Pat. No. 5,038,682 issued Aug. 13, 1991 discloses a remote controllable electronic detonator and a method of detonating an explosive charge. The detonator comprises an antenna, a RF receiver, an energy storage capacitor, a switch, a delay time circuit and a fuse. The method comprises the steps of transmitting to the detonator, by means of a transmitter, a wave comprising a carrier amplitude modulated by a low frequency modulating signal, receiving the wave and utilizing energy in the wave to charge a capacitor, enabling the switch by increasing the frequency of the modulating signal and communicating, by means of the wave, a fire command signal to the detonator. After a predetermined time delay, the switch connects the capacitor to the fuse thereby to energize the fuse.
In another example, International Patent Publication WO2003/029748, published Apr. 10, 2003, discloses a blasting system comprising a wireless link between a blast controller and a plurality of electronic detonators. Each detonator comprises a respective electronic initiator and an explosive charge. Charge storage devices of the initiators are chargeable by a carrier of a first signal having a first frequency in the order of 400 MHz-500 MHz and which is broadcasted by the blast controller. Each initiator further comprises logic circuitry driven by a clock signal which is derived from the first signal and having a clock frequency of about 4 kHz, which is substantially less than the first frequency.
Progress has been made in the development wireless detonator assemblies with internal safety features. Nonetheless, existing wireless blasting systems still present significant safety concerns, and improvements are required if wireless blasting systems are to become a more viable alternative to traditional “wired” blasting systems.